The hydrodynamics associated with instream large roughness elements in gravel-bed rivers

This thesis investigates the spatially distributed hydrodynamics associated with submerged large roughness elements (LREs) such as boulders, cobbles and pebble clusters in gravel-bed rivers. The specific objectives of this thesis are: (1) to identify and quantify the spatial scale-dependence of turbulent flow variables obtained from a vertical plane crossing over a LRE through the use of multivariate statistical techniques (2) to characterise the in situ 3D turbulent flow field with and without an isolated submerged LRE and investigate the hydrodynamic effect of LREs on large-scale (LS) turbulent flow structures advecting from upstream (3) to describe and quantify in detail LRE shear layers and associated shedding processes and the behaviour of meso-scale shedding structures under the influence of LS flow structures (4) to determine the effect of varied flow conditions (stage and mean longitudinal component velocity) and of LRE geometries (shape, size, orientation) on the in situ 3D turbulent flow field and turbulent momentum exchange.The main results of the thesis show that LREs generate intense 3D turbulence in the wake with peak values in the main turbulent indicators (e.g., turbulent kinetic energy, ke, and Reynolds shear stresses) 5 -- 10 times greater than free-stream values. These high values occur in a relatively localized narrow zone below the tops and directly downstream of the LREs. Wake turbulence was shown to be well structured by LRE size and mean free-stream longitudinal velocity which explained upwards of 70 -- 80% of the variance in ke, and longitudinal-vertical Reynolds shear stress, -- rhouw. A bimodal shedding process was observed in the lee of LREs where small-scale initial instability vortices intermittently amalgamated into larger meso-scale flow structures with shedding frequencies of approximately 5 Hz and 1 Hz, respectively. The shedding frequency of the meso-scale flow structures corresponded adequately to predicted values using a Strouhal number of 0.18. Elevated Reynolds shear stresses occurred in particular patterns in the wake of LREs. High longitudinal-lateral Reynolds shear stress -- rhouv occurred in downstream elongated +ve and --ve zones attributed to counter rotating vertical axis vortices shedding from the lateral sides of the LRE. High -- rhouw was observed to occupy much of the wake occurring over large scales Deltax = 5hs -- 10hs concurrently with high --- rhouv and often dominating the turbulent momentum exchange. Using the concurrent flow visualization the high -- rhouw and turbulent events were directly associated with lateral axis vortices shedding from the top of the LREs. The interaction between the shear layer structures and LS coherent flow structures was observed to be intermittent. LS flow structures were shown to be relatively unaffected by LRE shedding flow structures, while conversely the shear layer structures were seen to intermittently shed upwards and contract downwards towards the bed with the passage of LS ejections and sweeps, respectively. The fine-scale in situ hydrodynamics around LREs characterised in this thesis provide valuable insight on the role of LREs in gravel-bed rivers which will be helpful for the fields of fluvial geomorphology, river engineering and aquatic biology.Keywords. turbulence large roughness elements pebble clusters ADV visualization coherent flow structures variation partitioning spatial spectral space-time correlation multivariate analysis shear layer vortices aquatic habitat gravel-bed rivers. (Abstract shortened by UMI.)This thesis integrates the use of multiple complementary field sampling instrumentation techniques and statistical analyses to fulfil the outlined objectives and obtain a comprehensive characterization of the flow field around LREs in gravel-bed rivers. Multiple acoustic Doppler velocimeters (ADVs) were the primary instruments used for 3D high-frequency velocity measurements. Previously obtained 2D measurements from electromagnetic current metres (ECMs) were also analysed to address objective 1. A novel technique of concurrent ADV measurements with flow visualization was used to accomplish objective 3 permitting a detailed investigation of the LRE shear layer and associated flow structures. Multivariate statistical analyses were used to investigate scale relationships in the spatially distributed data as well as to find functional relationships between mean flow and LRE morphometric variables and turbulence variables estimated from the wake.